Okay, so, taking me back a few years...
The motor mounts should already be in the right place on the frame if you're going from a 250 to a small block chevy (they were on my '79). Same goes for the transmission crossmember if you're staying with either a turbo 350 or a metric 200 transmission. If you go to a 700R4 or a 4L60E, you'll need a different transmission crossmember and driveshaft.
The fan shroud will 'work' but it sits something like 3 inches further forward, so the fan will be exposed (not covered by the shroud) You can pull a shroud off of a newer V8 car, but most of them use a radiator that is 2-3 inches narrower. Other than that, you have exhaust to figure out, the shifter linkage is the same if you stay with column shift. You will need different radiator hoses. The fuel line should be the same, though I'd take the time to inspect it while you have the motor out. Will be easier to replace. Same goes for the brake line that runs from the proportioning valve to the passenger side front brake. Much easier to inspect or replace when the motor is out.
the Power steering lines will 'work' but they won't be happy. I kinda had to really twist mine around to fit properly, but they'll work in a pinch. I would also look up a vacuum diagram for a v8. There are a ton of 'emissions controls' you can rid yourself of, but important things, like the power brakes, vacuum advance on the distributor, and heater controls (if you have air conditioning, the switching is all vacuum operated)
One other thing: I noticed that if you get the motor from a truck, the motor mounts won't always line up perfectly side to side. I had a motor out of a suburban and the brackets that bolt to the motor were about 1/4 to 1/2 of an inch too wide side to side. Just something to look at. I think I ended up using a set of brackets off a full size Chevy Blazer.
Oh, and you'll want to take a little time reorganizing the wiring harness. all the important stuff is there, but things like the distributor is on the front of the motor on the L6, and at the back on theV8. The alternator is on the other side. Little things that can be a pain in the @$$ later on.
One more thing: If you stay with the original transmission, make sure you get a flexplate (flywheel) that has dual bolt patterns. The L6 uses a smaller diameter flywheel than the V8's, and thus has a smaller diameter bolt pattern on the torque converter. Most Small block chevy flexplates have both small and large diameter bolt patterns, but I've come across a few that don't. Just trust me, this is one of those things that you don't want to come across /after/ the motor is in the car. You can use the L6 flexplate on most pre 1986 small block chevy's (ones with the valve cover bolts along the perimiter) but NOT, I repeat NOT, on anything 1987 or later, (center bolt valve covers).
Anyhow, let me know any other questions you might have.